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How to avoid misdiagnosing common and uncommon conditions as a student


Many common and uncommon diagnoses mimic each other. Third-year student Shelby May offers her advice to help students diagnose patients correctly the first time.

The views expressed here belong to the author. They do not necessarily represent the views of Optometry Times or UBM Medica.


This blog derives from a side effect of the textbooks we are all buried in. I have mentioned in past blogs that I am in the midst of board studies. One of the sillier side effects of this is being attuned to unlikely diagnoses, or zebras.

Hearing hoofbeats“When you hear hoofbeats, think horses, not zebras” -Dr. Theodore Woodward

You may have heard the expression above. It is the honorary motto among medical students. The first day of optometry school, our theory professor introduced this concept. I cannot think of a class that I have had since that I have not heard this phrase. Despite all the times I have heard it uttered, I had never seen anyone actually cite the phrase.

Previously from Ms. May: 5 true pieces of advice first-years hate hearing

After a bit of research, it seems this quote was first attributed in the 1940s to Dr. Theodore Woodward of the University of Maryland. He used this quote to help keep his medical interns from over diagnosing the unlikely. Since then, it has been a gentle guide to the medical field. Horses are common diagnoses, while zebras are the not so common.
Why do we jump to the rare differential when diagnosing patients? The general consensus is that dangerous, improbable, and unique circumstances are easier to remember. That crazy case of vernal conjunctivitis caught your attention, but the next patient, a young boy with itchy red eyes, probably has allergic conjunctivitis.

Looking for stripes
One average clinic morning, a patient came in for his diabetic yearly exam. His blood glucose was poorly controlled, but he saw his physician frequently and she was happy with his health otherwise. I proceeded through the exam until the patient mentioned off hand that his vision “blurred-out” for a few seconds in both eyes a few days ago.

He had stood up very quickly while picking up a package, and the room dimmed for a few seconds before returning to normal. His retina looked healthy-no blood or ischemia. My staff doctor and I concluded it was likely orthostatic hypotension from simply standing up too quickly. We educated the patient on more concerning signs if the vision loss returned and sent him on his way.

That night I happened to study transient ischemic attacks (TIA) and carotid artery emboli. I reconsidered my patient’s case with sudden unease. Despite having already determined a rational cause for his vision dimming, I was thinking zebras.

Related: Technology helps to diagnose corneal ectasia

I emailed my staff doctor in a panic-I was worried that I had sent home a lit powder keg. After a few hours and a chat with a friend later in the day, I realized how ridiculous I sounded. Young, dimming vision in both eyes for a few seconds, and standing up quickly pointed to orthostatic hypotension, not an embolus.

I sent another email to my staff doctor that in many words amounted to, “Sorry to bother you. I got distracted by zebras.” She was kind enough to never mention the exchange in person. I was embarrassed for days.

Avoiding misdiagnosis
Since this incident, I use a couple of tricks to keep myself in line. I have doodled a horseshoe on the first page of my notebook that I keep in my coat pocket. It looks a bit silly, but I smile and remind myself every time I open my little book to look for horses.

Beneath the horseshoe are my Top 3 lists. They list common complaints and three common diagnoses behind each complaint. Each diagnosis has a few symptoms listed after it that separate this particular diagnosis from the others.

Experience is the best teacher. As time goes on, I have no doubt that we will have hands-on evidence of which differentials are most likely. Until then, my little lists keep me from running off on unlikely tangents.

Related: How registries can help optometry

If lists do not suit your fancy, I suggest having some sort of failsafe in place no matter how academically gifted you are. Something about the heat of the moment tends to throw academia out the window.

Zebras are rare, but need to be known
Though you may never encounter some of these zebras, you must always be prepared and aware of them. The concern is not fungal keratitis sweeping the nation; it is that fungal keratitis is tough to take care of, especially if it is confused for something else for a month.

If it is shaped like a horse and black-and-white striped, it is probably not a painted horse. It is a zebra, and you should start treating it like one. Somebody out there has Tolosa-Hunt syndrome, and if he sits in your chair tomorrow, you must to be aware and ready to provide the proper care.

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